I have problem with Mach3 that before that just run great. I have my 'Spindle speed averaging' not saved even I have to restart Mach3. The rest just saved like it should. How to fix this?And also if I don't select 'Spindle speed averaging', why the RPM reading not showed up in Spindle DRO/display? But I can get the spindle running through all speed.

You should be able to save a configuration. So select the spindle speed averaging ,apply, click ok. You need to turn the spindle on in order to have rpm show when it's running and properly configured in Mach.

Try changing the setting as I replied. Then go up to the tab and save settings. Then exit Mach and come back in. See if it now changed.

Not sure for the mill but for the lathe spindle speed averaging just takes initial and last rpm and averages them to base the next feed rate for threading.So practically speaking may not that important for the mill.